The SPX3819M5-L is a 500 mA low-noise LDO voltage regulator in a SOT-23-5 package, producing a fixed 3.3 V output (this exact ordering code is the 3.3 V option, marked G1WW) from an input of 2.5 V to 16 V. It offers ±1% output tolerance, 40 µVRMS output noise with a bypass capacitor, 70 dB typical PSRR, a logic-compatible enable pin, and reverse battery protection, current limit, and thermal shutdown. The datasheet explicitly targets battery-powered applications — cordless telephones, radio control systems, and portable computers — as well as low-power voltage-reference duty. It is a moderate-current, low-differential part: in SOT-23-5 the thermal budget, not the 500 mA rating, usually sets the real limit.
Below are the questions engineers actually ask when bringing this part into a design, answered from its own datasheet.
What is the SPX3819M5-L for, and when should I pick it? (What use cases does the datasheet name?)
The datasheet's General Description explicitly names battery-powered applications: cordless phones, radio control systems, and portable computers. The spec profile backs that up:
- Battery-powered portable devices. 90 µA typical quiescent current at no load, near-zero off-mode current (0.05 µA typ / 3 µA max when disabled), and a logic-compatible EN input make it easy to gate battery drain. The 16 V input ceiling tolerates higher cell stacks and unregulated sources.
- Low-noise analog and RF rails. With a 1 µF bypass capacitor on the BYP pin, output noise drops to 40 µVRMS (10 Hz–100 kHz, CIN=CL=CBYP=1 µF at 10 mA) — the datasheet markets the part for radio control systems, where supply noise matters.
- Precision reference duty. ±1% output tolerance and a 57 ppm/°C typical tempco let it double as a low-power voltage reference, per the datasheet.
- Loads up to ~500 mA — a small MCU, sensor, or RF front-end. Current limit is 800 mA typ / 950 mA max at a hard output short, so it is not a heavy power-rail part.
- Gated auxiliary rails. The 0.4 V disable threshold plus near-zero off-state current make it a clean switchable rail for sleep modes.
Fixed outputs across the family run 1.2, 1.5, 1.8, 2.5, 3.0, 3.3, and 5.0 V; the plain SPX3819M5-L is the 3.3 V option.
How do I integrate the SPX3819M5-L into a design? (Pinout, capacitors, and the EN pin)
Pinout (SOT-23-5):
| Pin | Name | Function |
|---|---|---|
| 1 | VIN | Supply input |
| 2 | GND | Ground |
| 3 | EN | Enable — logic high enables; logic low or open = shutdown |
| 4 | BYP | Internal reference bypass (fixed-output version) |
| 5 | VOUT | Regulator output |
The key fact in that table: on this fixed-output part, pin 4 is BYP, not a feedback divider — only the adjustable variant uses pin 4 as ADJ. And pin 3, EN, must be explicitly driven; a floating EN leaves the regulator off.
Capacitors:
- Input: 1 µF close to the IN pin is what the datasheet's own characterization uses; size it for the 16 V operating range (absolute max on VIN is ±20 V).
- Output — required, not optional. The datasheet states the part requires an output capacitor for device stability. A 2.2 µF aluminum electrolytic works in most circuits; 1 µF tantalum often gives the same stability. Stability margin shrinks as output current rises — a ~10 mA load needs only about half the capacitance of a 150 mA load. The datasheet explicitly recommends bench-testing the cap type/value because high-frequency electrolytic behavior varies by manufacturer.
- BYP (pin 4): a 10 nF cap significantly cuts output noise (from ~300 µVRMS unbypassed to ~50–60 µVRMS at 10–100 nF), but it can be left unconnected if noise isn't a concern. The trade-off: start-up speed is inversely proportional to BYP size, so omit it if you need fast turn-on, or use a larger value for a slow, controlled ramp.
EN pin: pull above 2.0 V to enable, below 0.4 V to disable; its operating range is 0 V to VIN. For an always-on rail, tie EN directly to VIN. For MCU-gated operation, drive it from a GPIO. Do not rely on an internal pull-up — the datasheet's "logic low or open = shutdown" wording means a dangling pin is off, and no internal bias is specified to guarantee otherwise.
Dropout budgeting: keep VIN at least ~0.5–0.7 V above VOUT if you plan to source near full load. At 500 mA the dropout is 340 mV typ / 550 mV max at 25 °C, and 700 mV max over temperature. Output tolerance is ±1% typ at 25 °C (±2% over range), with 0.04 %/V typ line regulation and 0.05 % typ load regulation — a tight rail when headroom is adequate.
What limits the SPX3819M5-L in practice? (Thermal budget and protection behavior)
The SOT-23-5 package has θJA = 191 °C/W — the worst of the three packages offered (NSOIC-8 is 128.4 °C/W, DFN-8 is 59 °C/W). With a −40 to +125 °C junction range, max allowable power is PD = (TJmax − TA)/θJA, which works out to roughly 0.52 W at 25 °C ambient and ~0.31 W at 65 °C. Exceeding it sends the part into thermal shutdown. Practically: at 5.0 V in, 3.3 V out, 300 mA load, the dissipation puts the junction near 122 °C at 25 °C ambient — right at the limit. Treat the M5-L as a moderate-current, low-differential package, and keep (VIN−VOUT) × ILOAD within ~0.3 W for typical indoor ambients.
On protection: reverse battery protection is a listed feature, so no external reverse-blocking Schottky is mandated. VIN and EN are rated −20 V to +20 V absolute max, with 2 kV HBM ESD; if your source can surge beyond that, add a TVS near IN. Current limit is ~950 mA at VOUT = 0 V, so a hard short folds back rather than destroying the pass device.
Known limitation: the datasheet does not publish a thermal-shutdown trip temperature or hysteresis, an output-capacitor ESR window, a numeric startup-time spec, or any quantitative reverse-output-current behavior for VOUT > VIN. Only the feature names (thermal shutdown, current limit, reverse battery protection) and the qualitative BYP/start-up relationship are documented. If your design depends on any of these numbers, characterize on the bench or ask the manufacturer.
How do I test an SPX3819M5-L circuit? (Verification procedure with pass/fail thresholds)
Measure directly on the VOUT pad, not the connector. Datasheet specs are taken at VIN = VOUT+1 V, CL = 1 µF, TA = TJ = 25 °C, so interpret against your actual conditions. A sensible sequence:
- Off-mode quiescent: drive EN below 0.4 V and measure input current — pass at ≤3 µA max (0.05 µA typ). Then float EN and confirm the rail stays off; this catches a missing EN tie on your board.
- Enabled no-load ground current: EN high, no load — pass at ≤150 µA max (90 µA typ).
- Output accuracy: within ±1% of nominal at 25 °C (±33 mV on 3.3 V), ±2% over full range.
- Line regulation: sweep VIN from VOUT+1 V to 16 V; output change ≤0.1 %/V max (0.04 typ).
- Load regulation: step 0.1 mA to 500 mA; output change ≤0.4% max (0.05 typ).
- Dropout: trim VIN down until VOUT sags 2%, then read VIN−VOUT against the limits — 60 mV at 100 µA, 175 mV at 50 mA, 350 mV at 150 mA, 550 mV at 500 mA (25 °C max); over temperature those become 80 / 250 / 450 / 700 mV.
- Current limit: short VOUT and confirm fold-back at 800 mA typ / 950 mA max, with recovery after removing the short.
- Thermal: run worst-case differential and load, measure case/ambient, and confirm (VIN−VOUT) × ILOAD × 191 °C/W added to ambient stays under 125 °C junction.
- Noise: if the rail feeds analog/RF, confirm ~300 µVRMS unbypassed or 40 µVRMS with CBYP = 1 µF (plus CIN = CL = 1 µF) at 10 mA, and that the BYP cap is actually fitted.
- Stability: apply your worst real load step (largest dI/dt) with the actual output cap on the board and watch for ringing or oscillation — this is exactly what the datasheet's bench-testing guidance is warning about.
Why is my SPX3819M5-L circuit misbehaving? (Troubleshooting by symptom)
- No output voltage: check EN first. "Logic low or open = shutdown" means a floating pin 3 leaves the part off. Confirm pin 3 is ≥2.0 V or tied to VIN, and that VIN is present within the 2.5–16 V operating range.
- Output a few hundred mV low / out of regulation: you're in dropout. Compare VIN−VOUT against the dropout table above (e.g. 550 mV max at 500 mA at 25 °C); raise VIN or cut the load.
- Output slightly off but stable: within tolerance, not a defect. ±2% over range means a 3.3 V rail at +66 mV over temperature is in spec.
- Oscillation or ringing on load steps: output capacitor problem. Verify the cap is present at the pin and try a higher-quality part — the datasheet requires an output cap for stability and warns electrolytic HF characteristics vary by manufacturer.
- Runs hot or output pulses/cycles: thermal shutdown cycling. (VIN−VOUT) × ILOAD exceeds what the 191 °C/W SOT-23 can dissipate; reduce the differential, the load, or the copper's thermal isolation.
- Noisy rail on a sensitive load: missing BYP cap. Noise sits near 300 µVRMS without one and drops to ~50–60 µVRMS once CBYP reaches 10–100 nF.
- Battery draining in sleep: EN isn't actually below 0.4 V. A mid-level EN voltage that neither meets the 2.0 V enable threshold nor the 0.4 V disable threshold is not clearly specified by the datasheet — drive EN fully low (≤0.4 V) to guarantee the 0.05 µA typ off-state instead of the 90 µA typ enabled ground current. A floating EN, per the datasheet, is shutdown — so if the rail is draining in sleep, suspect a mid-level drive rather than an open pin.
The four show-stoppers, in order of how often they bite: thermal power budget in the SOT-23, dropout headroom at hundreds of mA, a missing output capacitor, and a floating EN pin. All four are checkable in minutes with a multimeter and a scope.
Part page: SPX3819M5-L.